The present invention relates to a solid-state imaging device capable of preventing black reference level fluctuations by shaded pixels.
There is proposed a MOS type solid-state imaging device in which each pixel is provided with a light-receiving section having a photodiode or one or a plurality of MOS (metal-oxide semiconductor) transistors and a scanning circuit formed in the vicinity of this light-receiving section and which reads image data by the scanning circuit. FIG. 3 shows one exemplary circuit diagram of the solid-state imaging device in which the pixels are two-dimensionally arranged.
In FIG. 3, there are shown pixels 21 and vertical read control lines 22 to be driven by a vertical reading circuit 23. There are also shown output signal lines 24, which are connected to a common signal line 27 via noise canceler circuits 25 and horizontal selector switches 26 provided for each column. The horizontal selector switches 26 are driven by a horizontal read circuit 28. The reference numeral 29 denotes an amplifier.
FIG. 4 shows the detailed construction of the pixel 21. As the structure of the pixel 21, there are known a structure that has one photodiode and one MOS transistor (FIG. 4A), a structure that has one photodiode and three MOS transistors (FIG. 4B), a structure that has one photodiode and four MOS transistors (FIG. 4C) and so on. In these cases, there are shown a photodiode 31, a pixel select transistor 32, an amplify transistor 33, a reset transistor 34 and a transfer transistor 35. That is, in the pixel structure shown in FIG. 4A, only selection of a photodiode signal is performed. In the pixel structure shown in FIG. 4B, the photodiode signal is read after being amplified by the amplify transistor 33. In the pixel structure shown in FIG. 4C, sensitivity is improved by separating the photodiode 31 from the detector section by the transfer transistor 35.
In FIG. 3, a plurality of columns located on the left-hand side in a pixel array are covered with a shading layer 30. Therefore, a black reference level can be obtained from an output of the pixels (shaded pixels) 21 covered with the shading layer 30. That is, a video signal is generally outputted by AC coupling, and on this occasion, a reference level for outputted video signal is indispensable. The output of the shaded pixels provides this reference level.
Moreover, the transistors 32, 33, 34 and 35 in the pixel 21 are required to be as small as possible, and the substrate concentration under the transistors 32, 33, 34 and 35 are required to be increased for the above purpose. On the other hand, it is required to expand a depletion layer under the photodiode 31 as far as possible in order to improve the sensitivity of the photodiode 31. Then, the substrate concentration under the photodiode 31 must be reduced for the above purpose.
In order to satisfy the aforementioned requirements, there is proposed a CMOS (complementary type metal oxide film semiconductor) type solid-state imaging device in which a well of a concentration higher than that of the substrate is formed of a conductive type identical to that of the substrate on the lowly doped substrate, a transistor is formed on the highly doped well, and a photodiode is formed on the lowly doped substrate (Japanese Patent Laid-Open Publication No. HEI 11-307753). FIG. 5 shows the structure of this CMOS type solid-state imaging device. In this case, there are shown a lowly doped substrate 1, a highly doped well 2, a photodiode 3, a source or drain 4 of a transistor, and the gate of the transistor 5. A plurality of pixels, which are arranged on the left-hand side in the figure and constructed of transistors and the photodiodes 3 are covered with a shading layer 6, and an output signal from the shaded pixels covered with this shading layer 6 is served as the black reference level.
However, the conventional CMOS type solid-state imaging devices have the problems as follows. That is, as described above, in the CMOS type solid-state imaging device shown in FIG. 5, the level of the output signal from the shaded pixel covered with the shading layer 6 is served as the black reference level. In the above case, if intense spot light is made incident on some of the light-receiving pixels that are not covered with the shading layer 6, then part of light 7 reaches the neutral region of the lowly doped substrate 1. Then, a minority of carriers 8, which are photoelectrically converted there, out of electric charges diffuse and expand in all directions. Thus, part of these diffused charges 8 reaches the photodiodes 3 of the shaded pixels, consequently raising the output signal level of the shaded pixel. As a result, the output signal level of the shaded pixels becomes unable to serve as the black reference level.
If this phenomenon is observed on a pickup image screen, the result is shown in FIG. 6. In this case, FIG. 6A shows the pickup image screen. FIG. 6B shows an image signal in a portion LA–LA′ of the pickup image screen shown in FIG. 6A, where the horizontal axis represents a distance along LA–LA′, and the vertical axis represents the output level. As shown in FIG. 6A, there appears a false signal of a belt-shaped black debased image in the horizontal direction from the position of the spot light 9. As shown in FIG. 6B, the minority of carriers generated in the region of the intense spot light 9 diffuse and reach the neighborhood shaded pixels, pulling up the signal level located there. Normally, with regard to the video signal, the DC level of the light-receiving pixel signal is determined with respect to the shaded pixel signal per each horizontal line. Therefore, the level of the light-receiving pixel signal become reduced by the level of the shaded pixel signal in horizontal lines, and this cause a black debased image (hereinafter expressed as “depressed in black”) in a belt-like shape in the horizontal direction on the pickup image screen 6A.